Experience-rated plans

I am not an expert in Form 5500 but I am very curious about one question here: I thought all large groups (>50 enrollees) are experience-rated to some extent (premiums depending on their past claims). But why most Form 5500 Schedule A filings are nonexperience-rated? Is this an option that employers can choose? If yes, what kind of employers are more likely to file as nonexperience-rated? Thanks!

As I understand it (and I'm not an expert at all!) it's how the contract is arranged with the insurance carrier. So yes, it's a choice.  Here's an article I found that talked about experience-rating from the perspective I've seen it. Group insurance premiums are adjusted, not year over year, but based on comparison to another similar group. 

https://www.investopedia.com/terms/e/experience-rating-insurance.asp